1. Field of the Invention
The invention relates to a silicon-oxide-nitride-oxide-silicon (SONOS) device.
2. Description of the Prior Art
Non-volatile memory devices are currently in widespread use in electronic components that require the retention of information when electrical power is terminated. Non-volatile memory devices include read-only-memory (ROM), programmable-read-only memory (PROM), erasable-programmable-read-only memory (EPROM), and electrically-erasable-programmable-read-only-memory (EEPROM) devices. EEPROM devices differ from other non-volatile memory devices in that they can be electrically programmed and erased electrically.
Product development efforts in memory device technology have focused on increasing the programming speed, lowering programming and reading voltages, increasing data retention time, reducing cell erasure times and reducing cell dimensions. Some of the flash memory arrays today utilize a gate structure made of dual polysilicon layers (also refers to as the dual poly-Si gate). The polysilicon layer utilized in these gate structures often includes a dielectric material composed of an oxide-nitride-oxide (ONO) structure. When the device is operating, electrons are injected from the substrate into the bottom layer of the dual polysilicon layers for storing data. Since these dual gate arrays typically store only one single bit of data, they are inefficient for increasing the capacity of the memory. As a result, a flash memory made of silicon-oxide-nitride-oxide-silicon (SONOS) is derived. Preferably, a transistor from these memories is capable of storing two bits of data simultaneously, which not only reduces the size of the device but also increases the capacity of the memory significantly. The operation of a typical SONOS memory is described below.
During the programming of a typical SONOS memory, electrical charge is transferred from a substrate to the charge storage layer in the device, such as the nitride layer in the SONOS memory. Voltages are applied to the gate and drain creating vertical and lateral electric fields, which accelerate the electrons along the length of the channel. As the electrons move along the channel, some of them gain sufficient energy to become trapped in the charge storage dielectric material. This jump is known as hot carrier injection, in which the hot carriers being the electrons. Charges are trapped near the drain region as the electric fields are strongest near the drain. Reversing the potentials applied to the source and drain will cause electrons to travel along the channel in the opposite direction and be injected into the charge storage dielectric layer near the source region. Since part of the charge storage dielectric layer are electrically conductive, the charged introduced into these parts of the charge storage dielectric material tend to remain localized. Accordingly, depending upon the application of voltage potentials, electrical charge can be stored in discrete regions within a single continuous charge storage dielectric layer.
However, the ability for trapping and retaining electrical charges under current SONOS architecture is still not perfect, including shortcomings such as insufficient trapping sites for charges as well as easy leakage. Hence how to effective improve the current SONOS architecture to increase the overall performance of the device has become an important task in this field.